Recently, a circuit substrate comprising a metal foil having formed thereon an insulating layer composed of a polyimide resin has been used as a thin-film multilayer circuit substrate for the purposes of high-density packaging of semiconductors and a high-speed signal treatment. However, since the polyimide resin which is conventionally used as an insulating material has a large coefficient of linear thermal expansion, the circuit substrate is liable to cause cracking on the resin layer and warping and the delamination of the resin layer, and warping.
On the other hand, in computers and storage devices which are peripheral equipments for computers, in addition to the improvement of the capacity, small-sizing and a low cost have been required and with such requirements for a background, in particular, the technique of hard disk drives has been greatly advanced. Also, in the magnetic head, recently the developments of a thin-film magnetic head (TFH) wherein the coil portion is made of a thin film and further a thin film-magnetic resistance composite head (MR) which serves reading and writing and has a greatly large storage capacity have been hurried to a conventional metal in gap (MIG).
However, by a conventional technique of constituting desired wiring by laying a conductive wire on a suspension substrate, the conductive wire gives influences on the elastic modulus of the suspension to cause the fluctuation of the pushing force of the suspension and as the case may be, by the contact with the magnetic disk, the durability of the magnetic disk device is sometimes lowered.
Therefore, recently, a suspension formed by directly forming an electric circuit on a suspension substrate for mounting a magnetic head has been practically used. However, the suspension finally obtained using the conventional circuit substrate having a polyimide resin as an insulating layer also causes insulation failure and warping occur, resulting in performance failure in some cases, since the polyimide resin has a coefficient of linear thermal expansion larger than that of a metal foil substrate.